1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device. More particularly, the present invention relates to a method of fabricating a contact.
2. Description of the Related Art
In semiconductor manufacturing, photolithographic and etching processes are often used to pattern out contacts. As the level of integration continues to increase, the critical dimensions of each device is reduced. In other words, using photolithographic and etching processes to pattern out very small contacts is increasingly difficult.
On the other hand, very small contacts are required in some applications. For example, micro contacts are fabricated inside a chalcogenide memory unit.
Chalcogenide memory is a type of non-volatile memory. Chalcogenide is a special type of material that undergoes a phase change (a transition from an amorphous state to a crystalline state) when subjected to heat so that data can be written into or erased from the memory unit. In a chalcogenide memory unit, the area of contact between the chalcogenide material and the electrode is called a chalcogenide active region. The active region is located within a contact opening so that the dimension of the active region is the size of the contact opening. When a current pulse passes through the active region, a phase change of the chalcogenide material inside the active region occurs. In general, current density of the current pulse is related to the area of the active region. When the contact opening is small, the active region is small and hence a large current density is produced. If the current density is large, the current required to program data into the chalcogenide memory unit is small.
Thus, with an increasing level of integration and the importance of forming micro contacts in the aforementioned area, several methods for fabricating micro contacts have been developed.
One method of fabricating a micro contact has been disclosed in U.S. Pat. No. 6,111,264. In this method, spacers are formed inside a contact opening to reduce an overall dimension of the contact. However, the spacers are formed in photolithograhic and etching processes. Due to the limitation of the photolithograhic and etching processes in the patterning of micro features, a spacer with an uniform thickness is hard to produce and the ultimate reduction of the contact size is quite limited.
Another method of fabricating a micro contact has been disclosed in U.S. Pat. No. 6,031,287. In this method, a hard mask layer with small openings is formed to serve as an etching mask to form the micro contact opening. Because the process of forming the hard mask layer still relies on the formation of spacers to reduce the size of openings, problems similar to the aforementioned disclosure still exist.
Yet another method of fabricating a micro contact has been disclosed in U.S. Pat. No. 6,114,713. In this method, a photoresist layer is formed over the areas predefined to form contacts. Thereafter, the photoresist layer is used as an etching mask to perform an isotropic etching operation so that a micro contact is formed underneath the photoresist layer. However, the size of the contacts obtained through this method tend to be non-uniformed due to considerable difficulties in controlling the etching operation.
Although the aforementioned methods can reduce the average size of contacts, each method has some limitations. Therefore, a method for forming micro contacts with as little setbacks as possible is still a major area of investigation in semiconductor production.